1. Field of the Invention
The present invention relates to a negative feedback amplifier being suitably used for a high-speed optical communication system and more particularly to a negative feedback amplifier with automatic gain control function that allows automatic control of a gain.
2. Description of the Related Art
An optical communication system employs an optical-electrical converter used to convert an optical signal to an electrical signal. The electrical signal obtained from the optical-electrical converter such as a photodiode is amplified by a negative feedback amplifier. Such negative feedback amplifiers are disclosed in Japanese Laid-open Patent Application No. 10-284953. The disclosed negative feedback amplifier is comprised of an inverting amplifier circuit and a first feedback resistor connected in parallel to the same. In the above disclosed negative feedback amplifier, a second feedback resistor which can be connected in parallel to the first feedback resistor through a switching device. When a gain of the feedback amplifier is not more than a predetermined value, the switching device is in an open state, while, when the gain exceeds the predetermined value, the switching device is in a closed and connected state. If the gain of the feedback amplifier exceeds the predetermined value much and changes greatly, the switching device is so operated that its opening and closing operation serves to change impedance of the feedback amplifier, thus enabling the gain of the feedback amplifier to be maintained within a specified range.
When the second feedback resistor is connected to the feedback circuit to reduce the gain, due to a decrease in the impedance of the feedback circuit, frequency characteristics of the impedance extends. At this point, in the high frequency range, a peak may appear in the frequency characteristics of the impedance in some cases. In order to remove the peak in the high frequency range, in the disclosed negative feedback amplifier as shown in FIG. 3 of the publication, it is proposed that a by-pass condenser is additionally connected in parallel to the second resistor. By inserting such a by-pass condenser, the frequency characteristics are flattened while the impedance of the feedback circuit is in a lowered state, thus providing better amplifying characteristics.
However, in the above-mentioned conventional negative feedback amplifier, though one end of the by-pass condenser is interrupted from the inverting amplifier circuit due to interrupting actions of the switching device while the by-pass condenser is not in an active state, the other end of the by-pass condenser is connected to an output terminal of the inverting amplifier circuit. Because of this, parasitic capacity of the by-pass condenser is added to the inverting amplifier circuit, thus causing bad influence on the frequency characteristics of the negative feedback amplifier.